Some coder-decoder (CODEC) circuits require a voltage which is relatively close to the bandgap voltage of silicon and which remains essentialy constant over operating temperature ranges. One commonly used configuration for creating such a voltage consists of a first bipolar transistor with a current source connected to the emitter and in addition, second and third transistors with separate current sources coupled to each of the emitters. The magnitude of the current flowing through each of the second and third transistors, as well as the area of the emitters of each of these transistors, is carefully selected such that a differential output voltage occurs between the two respective emitters. One emitter is coupled to the plus input terminal of an amplifier, and the other emitter is coupled to a negative input terminal of the amplifier. The amplifier generates an output voltage which is characterized by an increase in magnitude level with increasing temperature. The output voltage at the emitter of the first transistor decreases in magnitude with increasing temperature. The gain of the amplifier is adjusted to cause the differential output voltage of the second and third transistors to be amplified such that when added to the output emitter voltage of the first transistor, the resulting output voltage is essentially constant over a useful operating temperature range. The gain of the amplifier is typically set using a pair of resistors whose ohmic value must be adjusted (trimmed) to achieve the required gain. This adjustment is undesirable in that it requires time and expense. The relative areas of the emitters of the second and third transistors and the ratio of the current therethrough can be adjusted to within limits to set the magnitude of the output emitter voltages, but it is typically not practical to vary the emitter areas and currents enough so that the output differential voltage generated is great enough to completely compensate for the output voltage variation of the first transistor. Thus, an amplifier having a preselected gain, which is achieved by trimming resistors, is used. Another problem with this type of circuitry is that the input offset voltage of the amplifier is also multiplied by the gain of the amplifier, and this must be corrected using other circuitry. Still another problem with this type of circuitry is that the output voltage of the first transistor has a somewhat curved slope and is difficult to completely compensate for. Accordingly, the output voltage of the circuitry, though relatively constant, is not as precise as may be needed in some applications.
It is desirable to have a voltage generator circuitry which does not require the trimming of resistors and in which the resultant output voltage is more constant with temperature variation than is available with much of today's existing circuitry.